Optical pickup devices for reading/writing an optical information disc have an objective lens actuator. Provided with an objective lens, the objective lens actuator moves the objective lens relatively to the optical information disc in a focusing direction and a tracking direction.
Recent optical information discs provide increasingly high date storage densities. Objective lenses for use in a read/write process for an optical information disc have increasingly large numerical apertures (NA).
When the optic axis of the objective lens inclines off. the normal to the recording surface of the optical information disc, the spot formed by light converged by the objective lens exhibits coma and astigmatism aberrations. Coma aberration increases in proportion to the cube of the numerical aperture of the objective lens. Astigmatism aberration increases in proportion to the square of the numerical aperture of the objective lens. Therefore, objective lenses with a larger numerical aperture allow a smaller tilt for the optic axis of the objective lens.
The optical information disc of high storage density is warped like a bowl in a write/read process. Accordingly, there is an increasing need to change the orientation of the objective lens so that the tilt be corrected, i.e., the optic axis of the objective lens be always normal to the disc's recording surface.
An objective lens actuator enabling such tilt correction is disclosed in, for example, Japanese published unexamined patent application 2002-216380 (Tokukai 2002-216380; published on Aug. 2, 2002). An objective lens actuator disclosed in Tokukai 2002-216380 employs a moving magnet drive method where a magnetic yoke, focusing coil, and tracking coil are fixed to a base, and a lens holder and drive magnet move together.
However, in the objective lens actuator implementing the moving magnet drive method, the heavy magnet is inevitably mounted to a movable assembly, adding to its weight and adversely affecting its response in focusing, tracking, and tilt correction. The excess weight of the movable assembly is an especially serious issue when, for example, a large magnet is used to improve the magnetic circuit.
Another issue with the objective lens actuator is undesired tilting of the movable assembly. The phenomenon can be caused by the attraction between the magnet mounted to the movable assembly and the magnetic yoke mounted to the fixed assembly, depending on the shape of the magnetic yoke.
An objective lens actuator employing a moving coil drive method is disclosed in, for example, Japanese published unexamined patent application 11-312327 (Tokukaihei 11-312327/1999; published on Nov. 9, 1999). FIG. 9 shows the structure of an objective lens actuator 100 disclosed in Tokukaihei 11-312327/1999. The objective lens actuator 100 has two focusing coils 115a, 115b. Assume in FIG. 9 that the focusing direction matches the direction of the optic axis of an objective lens 113, or z-axis, and the tracking direction matches the disc's radial direction, or y-axis.
The objective lens 113 is held by a lens hold member 114. The coreless focusing coils 115a, 115b are attached to the lens hold member 114, symmetric with respect to the zx-plane. On a side of the coils 115a, 115b is there provided a flat tracking coil 116 so that it is positioned symmetric with respect to the zx-plane. A yoke 117 and magnets 118a–118d forms a magnetic circuit so that the focusing coils 115a, 115b and the tracking coil 116 are positioned in a gap. The A yoke 117 and magnets 118a–118d are each composed of iron or another magnetic material. The magnets 118a–118d produce a magnetic field.
The mutually facing magnet pair 118a, 118b is aligned so that the magnetic poles are in the same directions; so is the other mutually facing magnet pair 118c, 118d. The adjacent magnet pair 118a, 118c is aligned so that the magnetic poles are in opposite directions; so is the other adjacent magnet pair 118b, 118d. 
The objective lens actuator 100 torques the lens hold member 114 by means of the difference between the electric currents in the focusing coils 115a, 115b and moves the lens hold member 114 for focus adjustment by means of the sum of those electric currents. The objective lens actuator 100 hence provides tilt correction and focusing by a moving coil drive method, using the focusing coils 115a, 115b. 
Problems arise, however, because the objective lens actuator 100 in FIG. 9 uses the focusing coils 115a, 115b commonly for both focusing and tilt correction. A difference between the electric currents in the focusing coils 115a, 115b supplied from respective drivers will produce an asymmetric focus drive force, thereby undesirably tilting the lens hold member 114.